Colorimetric and Fluorometric Discrimination of Geometrical Isomers

Aug 6, 2015 - Soham Samanta, Chirantan Kar, and Gopal Das*. Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India...
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Rapid colorimetric and fluorometric discrimination of geometrical isomers (Maleic acid vs. Fumaric acid) with realtime detection of Maleic acid in solution and food additives Soham Samanta, Chirantan Kar, and Gopal Das Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b02202 • Publication Date (Web): 06 Aug 2015 Downloaded from http://pubs.acs.org on August 12, 2015

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Rapid colorimetric and fluorometric discrimination of geometrical isomers (Maleic acid vs. Fumaric acid) with real-time detection of Maleic acid in solution and food additives Soham Samanta, Chirantan Kar and Gopal Das* Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India. Fax: + 91 361 2582349; Tel: +91 3612582313; E-mail: [email protected]. ABSTRACT: Hetero-bis imine Schiff base probe L is able to discriminate geometrical isomers (Maleic acid vs. Fumaric acid) through sharp colorimetric as well as fluorogenic responses even conspicuous through naked eye. Colorimetric as well as fluorogenic sensing of Maleic acid among various carboxylic acids was also demonstrated in ethanol-buffer medium. Sensing behavior of L was corroborated by 1H-NMR spectra, mass spectrometry and theoretical calculations. Subsequently sensing behavior of L was used to probe maleic acid in starch rich food samples.

■ INTRODUCTION Molecular recognition of di-carboxylic acids has grabbed the limelight due to their presence as key structural moieties in many bioactive molecules along with their involvement in the biosynthesis of some important intermediates.1 Moreover, they have some important roles in various metabolic processes.2-3 Among these several carboxylic acids, Fumaric (Fum) and maleic (Mal) acids are the two important geometrical isomers having vast biological impacts, viz. they are extensively used in medicine, food and polymer industries.2-3 Recently, some advancement in using fumaric acid derivatives for the treatment of multiple sclerosis and patients with psoriasis are reported.4-7 Maleic acid, plays important role as an inhibitor of the Krebs cycle whereas fumarate is generated in the Krebs cycle. Excessive consumption of maleic acid found to be detrimental to kidney and can cause several kidney diseases.8-10 The widespread use of these two isomeric acids as ingredients in food as well as beverages raises a great concern over their adverse influences on human health upon prolonged exposure. Thus it is very important to develop an efficient chemosensor for their recognition and quantitative estimation in aqueous medium. Compared to the large number of chromogenic receptors for the sensing of metal ions,11-13 very few chromogenic receptors for anions and small molecules have been developed that are based on recognition approaches.13-16 In fact, a thorough literature survey revealed that there are very few colorimetric or fluorogenic sensors which dealt with the use of synthetically constructed receptors in carboxylic acids sensing purpose.17-18 Supramolecular concepts19-20 in designing new optical chemosensors has become a very common trend in recent-past based on changes in fluorescence21-23 and in absorbance24-26 as the output signals. In most of the cases the fluorescent probes are abiotic supramolecular systems that produce certain fluorogenic changes upon binding to the analytes by non-covalent interactions, such as hydrogen bonding, electrostatic attractions and coordination phenomena.27-28But in most of the cases these receptors encountered several limitations in terms of poor specificity towards the analytes.29-38 Differentiation of isomers is, in general, a difficult task because of their rather similar chemical and physical properties. A chemodosimetric reagent was reported by Manez and coworkers which could discriminate between maleate and fumarate ions.39 But this probe had also some limitations as

the selectivity of the system was poor and the probe rendered similar optical response in presence of phthalate if tested instead of maleate. In this context, as an advancement of this field, in our continuing effort to design optical sensors for biologically important analytes,40-41 herein we report a chromogenic system which not only can discriminate geometrical isomers (Maleic acid vs. Fumaric acid) but also can sense Maleic acid rapidly among various carboxylic acids through sharp colorimetric as well as fluorogenic responses in both physiological condition and food additives. ■ EXPERIMENTAL SECTION: General Information and Materials. All the materials for synthesis were purchased from commercial suppliers and used without further purification. The absorption spectra were recorded on a Perkin-Elmer Lamda-25 UV−Vis spectrophotometer using 10 mm path length quartz cuvettes in the range of 250−800 nm wavelength, while fluorescence measurements were performed on a Horiba Fluoromax-4 spectrofluorometer using 10 mm path length quartz cuvettes with a slit width of 3 nm at 298 K. The mass spectrum of the ligand L was obtained using Waters Q-ToF Premier mass spectrometer. NMR spectra were recorded on a Varian FT-400 MHz instrument. The chemical shifts were recorded in parts per million (ppm) on the scale. The following abbreviations are used to describe spin multiplicities in 1H NMR spectra: s = singlet; d = doublet; t = triplet; m = multiplet. Synthesis of the probe L. 10mmol (1.72g) of 2-hydroxy-1naphthaldehyde was dissolved in 50 mL of methanol and excess of hydrazine hydrate (5.0 mL, ~100 mmol) was added to this solution. The mixture was allowed to stir for 24 h to obtain pale yellow colour solid product which was filtered and washed thoroughly with cold methanol. This product constituted the Schiff base having mono-imine bond and a free amine group of hydrazine (scheme 1). In the next step, 1.0 mmol of this mono-imine product was dissolved in MeOH and 1.1 mmol of 4-(Dimethylamino)cinnamaldehyde was added. Resultant mixture was refluxed for 18 h to yield an orange solid. The product was washed with methanol and then dried in a desiccator.

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the following procedure. First 10g of starch rich food was taken in a 500 ml beaker and to it 200 ml of methanol was added and the mixture was stirred vigorously for 24 hrs. The extract was filtered and the filtrate was collected. From this, 27 ml of the filtrate was taken and and 3 ml HEPES buffer (7.4 pH) solution was added to it and then mixed well to get a homogeneous solution. Then this 30 ml solution was divided into three parts each containing 10 ml of solution. Among these three, in two parts maleic acid and fumaric acid were added (4mM) respectively from outside. Now the three 10 ml solutions containing none, maleic acid and fumaric acid were subjected to the spectral study. Scheme 1: Synthesis of probe L.

Calculated yield: 74%. 1H NMR [400 MHz, CDCl3, TMS, J (Hz), δ (ppm)]: 13.40 (1H, s), 13.00 (1H’, s), 9.64 (1H, s), 9.55 (1H’, s), 8.41 (1H, d, J=9.6), 8.10 (1H, d, J=8.4), 7.80 (1H, d, J=9.2),7.75 (1H, d, J=8.0), 7.52 (1H, t, J=7.2), 7.427.31 (2H, m), 7.22(1H, s, CDCl3),, 7.19 (1H, d, J=8.8), 7.06 (2H, d, J=15.6), 6.91-6.85(2H, m), 6.681(2H, d, J=9.2), 3.00 (6H, s). 13C NMR [100 MHz, CDCl3, TMS, δ (ppm)]:164.19, 160.85, 160.13, 159.43, 145.01, 134.03, 133.01, 129.19, 129.08, 128.13, 127.73, 123.68, 123.52, 120.06, 119.79, 119.16, 111.98, 108.55, 40.158. ESI-MS (positive mode, m/z) Calculated for C22H21N3O: 344.1685. Found: 344.1755 [(M+H+)]. UV−Vis and fluorescence spectroscopic studies: Stock solutions of various carboxylic acids (1 × 10−1 mol L−1) were prepared in methanol. A stock solution of L (5 × 10−3 mol L−1) was prepared in DMSO. The solution of L was then diluted to 20 × 10−6 mol L−1 with EtOH/aqueous HEPES buffer (5mM, pH 7.4; 9:1, v/v) for spectral studies by taking only 8 µL stock solution and making the final volume 2mL adding EtOH/aqueous HEPES buffer. In fluorescence titration experiments, a quartz optical cell of 1 cm path length was filled with a 2.0 mL solution of L (20 × 10−6 mol L−1) to which the carboxylic acid stock solutions were gradually added using a micropipette. In UV-Visible titration experiments, a quartz optical cell of 1 cm path length was filled with a 1.0 mL solution of L to which the carboxylic acid stock solutions were gradually added using a micropipette. In selectivity experiments, the test samples were prepared by placing appropriate amounts of the carboxylic acids stock into 2.0 mL of L solution (20 × 10−6 mol L−1). For fluorescence measurements, excitation was provided at 450 nm, and emission was acquired from 470 nm to 750 nm for the discrimination of maleic and fumaric acid and then excitation was provided at 550 nm, and emission was acquired from 570 nm to 800 nm for the sensing of Maleic acid. Detection Limit: The detection limits were calculated on the basis of the fluorescence titration. The fluorescence emission spectrum of L was measured 10 times, and the standard deviation of blank measurement was achieved. To gain the slope, the fluorescence emission at 606 nm was plotted as a concentration of maleic acid. The detection limits were calculated using the following equation: Detection limit = 3σ/k (1) where σ is the standard deviation of blank measurement, and k is the slope between the fluorescence emission intensity versus [maleic acid]. Detection of maleic acid in food additives: Sample for the detection of maleic acid in starch rich food was prepared by

■ RESULTS AND DISCUSSION A new hetero bis-imine fluorogenic probe L was synthesized (scheme 1) with good yield and in pure form. The probe was purposely designed to have a push-pull chromophoric platform along with protonation as well as H-bonding site for effective molecular recognition. The UV-Visible and fluorescence spectral properties of L were studied in detail to investigate the sensing behavior of L towards various organic acids. UV-Visible spectral study. UV-Visible spectra of L in 9:1 ethanol-HEPES buffer (5mM, pH~7.4) solvent revealed a sharp absorbance maximum at 430 nm due to π−π∗ transition. Binding selectivity of L towards different carboxylic acids was investigated by studying UVVisible spectra of L in presence of various mono and dicarboxylic acids viz. Acetic acid, Tartaric acid, Benzoic acid, Malonic acid, Succinic acid, Cinamic acid, Citric acid, Stearic acid, Glutamic acid, Phthalic acid, Teraphthalic acid, Isophthalic acid, Gallic acid, Succinic acid, Fumaric acid and Maleic acid. It was astonishing to note that in presence of excess (200 equivalents) of these carboxylic acids, L rendered no significant change in its colorimetric behavior except for maleic acid (Figure 1A). Addition of excess of Maleic acid to L in mixed solvent resulted in the generation of a new red shifted absorbance maximum at 552 nm with the subsequent decrease in the absorbance of its prior original maxima at 430 nm (Figure 1B). This unique UV-Visible spectral outcome was accompanied by a distinct visual change in the color of the experimental solution from pale yellow (only L) to dark red (L+Maleic acid) (Figure 1C). This visual display of the color change is encouraging for the naked eye detection of maleic acid using L through a colorimetric response. To understand this unconventional sensing of maleic acid by L, it is important to draw a quantitative appraisal of the relation between the spectral change and the equivalents of maleic acid interacted with L. Thus we carried out an UV-Visible titration experiment, where we found that incremental addition of Maleic acid to the solution of L resulted in a systematic growth of the new red shifted absorbance maxima at 552 nm, with a regular decline of the original absorbance maxima of L at 430 nm (Figure 1B). It is to be noted from the titration spectra that a clear isosbestic point arose at 495 nm. Interestingly no such optical changes were furnished by its isomer, fumaric acid in similar condition. Hence, the probe L can not only sense maleic acid among various other carboxylic acids but can also distinguish it from its geometrical isomer fumaric acid through a stupendous colorimetric response which is even conspicuous by naked eye.

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Figure1: A) UV-Visible spectra of L (20 µM) in presence of various carboxylic acids 4×10-3 M in 9:1 ethanol-HEPES buffer (5mM, pH~7.4). B) UV-vis spectra of L (20 µM) in presence of varying concentration of Maleic acid. INSET: Changes in the absorbance at 552 nm with addition of equivalents of maleic acid C) Visual color change of L in presence of different carboxylic acids in daylight. D) Comparison of the absorbance at 552nm of L on interaction with various carboxylic acids.

A bar graph has been plotted to show the exact changes in absorbance of L at 552 nm when treated with different carboxylic acids (Figure 1D) and this bar graph also clearly support the distinguished colorimetric feature in case of maleic acid. Preliminarily, we presumed that coordination of maleic acid with L through strong hydrogen bonding facilitated by cispositioning of two acid groups of maleic acid over other tested carboxylic acids led to the chromogenic spectral response alike earlier reported many carboxylic acid sensors. But, it was astounding to observe that addition of excess strong mineral acid like hydrochloric acid to L also generated a new absorbance maximum at 552 nm similar to that of maleic acid (Figure S5, Supporting information). Although unlike the case of maleic acid, actual absorbance peak of L at 430 nm was almost obliterated here. Hence there is a possibility that a new protonated species has formed, which led to the colorimetric response at 552 nm. To better explore the selectivity and sensitivity of L toward maleic acid we have perused a detailed fluorescence study, followed by mass spectral analysis, 1H NMR experiment and theoretical study for unraveling the exact sensing mechanism. Fluorescence spectral study. On excitation at 450 nm L renders a sharp emission maximum around 590 nm at the same experimental condition and emits a strong orange-yellow fluorescence (Figure 2). Interestingly addition of maleic acid only yielded significant change in the fluorescence spectra, whereas it geometric isomer fumaric acid has hardly any effect on emission of L in similar experimental condition. Addition of maleic acid induced a slight decrease in the fluorescent intensity of L with about ~15 nm red shift in the emission maximum to render a new emission maximum at 606 nm while interaction of fumaric acid with L remain quiet to the spectral change (Figure 2). It can be mentioned here that similar fluorescence experiment was carried out in pure ethanol in presence of maleic and fumaric acid, which renders similar optical response (Figure S6, Supporting information). These observations ensured that distinction of these two geometrical isomers is independent of buffer capacity and medium. However it may also be noted that addition of strong mineral acid HCl to the receptor L in ethanol medium resulted in substantially quenching (70%) of the fluorescence

of L to make it very less fluorescent unlike the case of Maleic acid (Figure S6, Supporting information). UV-Visible spectral study suggested that addition of maleic acid to L might lead to the formation of a new species as a new absorbance maximum emerged at 552 nm (Figure 1). Therefore, it was prudent to study the emission behavior of L in presence of maleic acid with an excitation wavelength of 550 nm.

Figure 2: Fluorescence spectra of L (20µ µM) in presence of 200 equivalents of Maleic acid and Fumaric acid in 9:1 ethanolHEPES buffer (5mM, pH~7.4). λex = 450 nm. INSET: Visual changes observed for L in absence and in presence of Maleic acid and Fumaric acid under UV light.

However, L showed a very weak fluorescence when excited at 550 nm (Figure 3). Selectivity of sensing was confirmed by studying fluorescence spectra of L (λex=550 nm) in presence of various mono and di-carboxylic acids as mentioned earlier. The change in the emission spectral properties was at par with the results of absorbance spectra.

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Figure 3: A) Fluorescence spectra of L (20 µM) in presence of various carboxylic acids 4×10-3 M in 9:1 ethanol-HEPES buffer (5mM, pH~7.4); λex = 550 nm. B) Fluorescence spectra of L (20 µM) in presence of varying concentration of Maleic acid; λex = 550 nm. INSET: Changes in the emission intensity at 606 nm with addition of equivalents of maleic acid. C) Visual color change of L in presence of different carboxylic acids and HCl under UV light. D) Comparison of the normalized emission intensity of L at 606 nm on interaction with various carboxylic acids (1= Fumaric acid, 2= Maleic acid, 3= Tartaric acid, 4= Citric acid, 5 = Cinamic acid, 6= Malonic, 7= Acetic acid, 8= Benzoic acid, 9= Gallic acid, 10= Stearic acid, 11= Terephthallic acid, 12= Phthalic acid and 13= Succinic acid).

Among all tested carboxylic acids, only maleic acid revealed a certain enhancement in the fluorescence intensity of L with a well-defined emission maximum at 606 nm (Figure 3A). Moreover, a selective conspicuous display of the pink-red fluorescence response of the probe upon interaction with maleic acid compared to the orange-yellow fluorescence display of L upon interaction with all other carboxylic acids provided the scope for naked eye detection (Figure 3C). A bar graph plot of the comparative normalized emission intensity of L at 606 nm, on interaction with various carboxylic acids also clearly indicates the exceptional selective turn-on florogenic response of L towards Maleic acid (Figure 3D). Thus, not only L could discriminate maleic acid from its isomer fumaric acid, but also selectively sensed it among various carboxylic acids through unique fluorescence as well as colorimetric responses. It may be highlighted here that selectivity of our probe L is superior compared to the probe reported by Manez and co-workers39 as probe L does not respond to phthalic acid at all in our sensing system. To get a quantitative appraisal of the spectral change a titration experiment was carried out with varying concentration of maleic acid. A systematic incremental addition of maleic acid to the solution of L in in 9:1 ethanol-HEPES buffer (5mM, pH~7.4) medium resulted in a sequential enhancement in its emission intensity at 606 nm when excited at 550 nm (Figure 3B). Interestingly the linear enhancement of the emission intensity of L up to addition of 360 equivalents of maleic acid downplayed the possibility of simple coordination between L and maleic acid. Instead both the UV-visible and fluorescence titration spectra indicated towards the possibility of a new species formation via protonation, which led to the unique spectral outcome. It may be mentioned here that the detection limit for the maleic acid was determined from this fluorescence titration experiment using equation 1, and it was found to be 2.1×10−5 M or 2.4 ppm (Figure S7, Supporting information ). There is a possibility that, the proton acceptor site -NMe2 group present in L could easily be protonated to generate a new species in solution. A close look of the acidity constant

value revealed that among all of the tested carboxylic acids maleic acid had the lowest pKa1 value (pKa1=1.9 Table S1) compared to its isomer fumaric acid (pKa1=3.03) which in turn facilitated easy proton generation in solution in the case of maleic acid. Thus maleic acid only could generate selective optical response through protonation. As discussed earlier, HCl could also generate a new absorbance maximum at 552 nm similar to maleic acid; however, it resulted in the formation of pink-red solution in contrast to the generation of dark brown-red solution in presence of maleic acid. Also the noticeable new feature was that, though in case of HCl the newly generated maximum was more prominent with much higher absorbance, the previous actual absorbance peak of L at 430 nm became almost nonexistent (Figure S5, Supporting information) unlike the case of Maleic acid. The emission spectral studies in presence of HCl also incited some major distinct spectral features with respect to maleic acid (Figure S6, Supporting information). These results substantiate that though the actual sensing mechanism was someway inclined by the protonation of the probe, but this is not the sole criterion for sensing mechanism. It maybe envisaged that some other factor, along with protonation is simultaneously governing the Maleic acid sensing. Protonation with subsequent complexation of the analyte(s) to the probe may be accountable for the whole sensing mechanism. It may also be noted that, oxalic acid, which has even lower pKa1 (1.25) value than maleic acid pKa1 (1.9) only could induce 65% less emission intensity (Figure S8, Supporting information) with respect to the enhancement in emission intensity initiated by maleic acid in ethanol. This observation again reiterated the preposition that complexation too has a role along with protonation to endorse the maleic acid sensing. Thus to uncover the precise mechanism behind this spectral change we pursued extensive 1H NMR and mass spectral experiments. 1H NMR study of L revealed that the presence of maleic acid led to the protonation of L (-NMe2 group) by signaling the appearance of a new peak at ~6.38 ppm (Figure S9, Supporting information). At the same time the almost

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disappearance of the phenolic −OH peak at ~13.4 ppm or ~13.0 ppm may suggest the strong complexation of maleic acid with successive de-protonation of the phenolic −OH. In addition, the mass spectra of the receptor L taken in presence of maleic acid witnessed the emergence of a new base peak at m/z=627. 6553. The peak corresponds to [L + 2Malate+ 3H2O+ H+], which clearly validates the protonation of L as well as coordination of maleic acid with L (Figure S10, Supporting information). The whole sensing phenomenon has been summarized and presented with a schematic representation in scheme 2. In contrary, addition of HCl to L yielded a NMR spectra indicating breaking of Schiff base linkage in the probe (Figure S11, Supporting information). Moreover the mass spectra of L taken in presence of HCl witnessed the appearance of mass peaks at 157.0385, 225.1048 and 347.2257. All these together pointed towards the disintegration of heteroSchiff base L and generation of a homo bis-imine (Figure S12, Supporting information). Probably the generation of the 4-(3-hydrazonoprop-1-en-1-yl)-N,Ndimehtylbenzenaminium led to the UV-Vis spectral outcome when strong inorganic acid has been added to L. Further a control experiment was carried out by studying the UV-Visible spectra of L in presence of maleic acid and HCl respectively followed by addition of excess NaOH to both the solutions (Figure S13, Supporting information).

mized geometry and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of L and protonated L (LH+) are presented in Figure 4. The substantial decrease in the HOMO to LUMO energy gap on protonation of L undoubtedly validated the theoretical basis of the observed red shift (Figure 2) in emission maxima as well as generation of new red shifted absorption maxima (Figure 1) of L when treated with maleic acid (Figure 4). UV-Vis spectra of L and LH+ were calculated using the TDDFT method (No specific solvent model was used) with the same B3LYP/6-31+G(d,p) basis set. The results of the calculation indicated that in the case of L, the transition from HOMO to LUMO, HOMO-2 to LUMO and HOMO-1 to LUMO / HOMO to LUMO+1 contributed mainly to the excitation at 427 nm, 412 nm and 360 nm respectively. Similarly, for mono-protonated L (LH+), absorption peaks in the long wavelength region at 544 nm, 520 nm, 420 nm and 403 nm were mainly generated due to the transition from HOMO to LUMO, HOMO-1 to LUMO, HOMO-2 to LUMO and HOMO to LUMO + 1 respectively. Selected orbitals and their corresponding energies for L and LH+ which are likely to be critical in the optical spectral outcome are provided in supporting information (Table S2 and Table S3).

Scheme 2: Plausible sensing mechanism

The result showed that subsequent addition of NaOH could retrieve the actual spectra of L in case of Maleic acid (Figure S13, Supporting information). However, it failed to salvage the absorbance peak of L at 430 nm in case of HCl. In the later case, formation of two distinct new absorbance peaks at 370 nm and 450 nm (Figure S13, Supporting information) is actually insinuating towards the breaking of imine bond of L and generation of two different new specie in solution in accordance with the earlier predicted mass spectra (Figure S12, Supporting information). Density functional theory (DFT) study. An extensive Density functional theory (DFT) and Time dependent Density functional theory (TDDFT) calculations were pursued to understand the theoretical aspect of this sensing mechanism. To ascertain the effect of protonation on spectroscopic signatures of L, DFT optimizations of L and protonated-L were carried out with the B3LYP/6-31+G(d,p) method basis set using the Gaussian 03 program. The opti-

Figure 4: Frontier molecular orbital plots, optimized structures and energy level diagrams of L and LH+. The calculations were performed using B3LYP/6-31 G (d,p) as implemented on Gaussian 03.

The calculated absorption peaks of L and LH+ found to be matched impressively with the experimentally observed peaks. Likewise the experimental UV visible spectrum of L having an absorbance maximum at 430 nm, theoretically calculated spectrum of L showed excitation at 427 nm, which is in remarkably well agreement. Alongside calculated UV-visible spectra for L witnessed emergence of a new excitation peak at 544 nm that agreed well with the newly generated absorbance maximum of L at 550 nm on interaction with Maleic acid (Figure S14, Supporting information). Not only that the theoretical and experimental spectral natures of

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L / LH+ are very similar which have been demonstrated in figure S14, (Supporting information). Thus undoubtedly lower pKa1 of Maleic acid assisted the mono-protonation of L along with coordination which governed to the observed unprecedented sensing outcome of Maleic acid by L. Determining Maleic acid in food additives. Use of Maleic acid as food additives in starch rich foods like tapioca starch, tapioca balls, rice noodles, and hotpot ingredients are not desierable.42-44 It was envisaged that the use of probe L to detect Maleic acid quantitatively in a starch rich food sample would be an encouraging real application. Thus we prepared a sample by simple liquid extraction of the starch rich food which is described in detail in experimental section. The extract of the food in methanol was divided in three parts and in each part maleic acid, fumaric acids and nil (as control) was added respectively in appropriate amount. Rising above our expectation L revealed the same spectral outcome as earlier and hence was able to detect maleic acid both calorimetrically as well as fluorometrically with the same detection limit. Thus we have developed a probe which could well be used to detect maleic acid in starch rich food samples optically (both fluorescence and UV-visible spectroscopy) even conspicuous through naked eye. ■ CONCLUSION In summery here in we have synthesized a new Schiff base probe L with two hetero-imine bonds, which is capable of discriminating Maleic acid from its geometrical isomer Fumaric acid through colorimetric as well

Figure 5: Determination of Maleic acid (spiked) in starchrich food A) from Fluorescence spectroscopy B) from UVVisible spectroscopy.

as fluorescence responses. Not only that, it can also sense the maleic acid among various mono, di and tri-carboxylic acids in mixed aqueous ethanol-buffer solution. The unique colorimetric as well as fluorescence responses of L towards Maleic acid is also conspicuous through naked eye. Theoretical DFT and TDDFT calculation well supported the premise that protonation of L is actually governing the sensing process along with complexation. The sensing process was also replicated in starch rich food to detect added Maleic acid. ASSOCIATED CONTENT Supporting Information. It contains 1H NMR spectra, 13C NMR spectra, mass spectra, UV−Vis spectra, Fluorescence spectra, Fluorescence Intensity vs. concentration of maleic acid plot, DFT/TDDFT calculations, acidity constant values table

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etc. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Prof. Gopal Das; Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India. Fax: + 91 361 2582349; Tel: +91 3612582313; E-mail: [email protected].

ACKNOWLEDGMENT G.D. acknowledges CSIR (01/2727/13/EMR-II) and Science & Engineering Research Board (SR/S1/OC-62/2011), India for financial support, CIF IITG for providing instrument facilities. SS and CK acknowledge IIT Guwahati for fellowship.

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